perf python: Avoid duplicated code in get_tracepoint_field

The code replicates computations done in evsel__tp_format, reuse
evsel__tp_format to simplify the python C code.

Reviewed-by: Howard Chu <howardchu95@gmail.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/20250228222308.626803-9-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>